D+B is one the thirteen brands in The World Ends With You. It's name stands for "Dangerous Buffalo," and is meant to represent the Cow of the Chinese Zodiac. These pins focus primarily on the Psychokinesis and Piercing Pillar pins.
It's implied that Shiki's favorite brand is D+B, because her clothes somewhat resemble the ones you may purchase there and "Mr. Mew" is also bought here. However, unlike the other protagonists, whose brand allegences are fairly cut-and-dry, Shiki's is bit more complicated - since her clothes are hand-made, they aren't actual D+B clothing.